Monday, March 15, 2010

Fern Species of Corkonian Limestone Walls

One of the most aesthetically pleasing aspects of Cork city and its environs are its numerous limestone walls. Many buildings of note up to the early 20th century were constructed or façaded with locally quarried limestone and may have limestone walls surrounding them. Even the river Lee as it passes through the city is bordered, for the most part, by limestone quay walls. These walls are also mortared with limstone. All of this mimics perfect habitats for plants that thrive on a calcareous substrate. Due to little if any organic matter, most flowering plants decline these walls in favour of the dirt-rich cracks where they meet the footpath. Often not even this is an option as the built up nature of the environment limits the light available. These conditions do lend themselves to an often perfect environment for certain fern species.

Limestone wall with Polypodium vulgare and Asplenium ceterach

The most frequent to be seen is the Common Polypody (Polypodium vulgare). This tends towards the upper third of walls, especially the top which it may often cover uninterrupted for some distance. Their bright green colour is complemented by vibrant yellow sori which ripen from July onwards. Common Polypody, along with the other species mentioned here,

Polypodium vulgare

Walls in the city centre contain three other fern species. Hart’s Tongue Fern (Asplenium scolopendrium) is often seen, but with a much reduced frond size from samples growing in areas of higher organic matter content, such as hedgebanks and ditches. Whereas fronds in these conditions can grow as much as 60 cm, on limestone walls they tend to between 10 – 20 cm. This reduction in size does not affect dispersal however, and once established populations will persist.

Asplenium scolopendrium

The species that tends to have the largest individual size is the Maidenhair Fern (Asplenium trichomanes). The prostrate growth habit of the fronds combined with their large relative length (up to 35cm on limestone walls) and tough, black stalks means this can often be the most evident fern on a wall.

Asplenium trichomanes

Probably the least conspicuous fern is the delicate Wall Rue (Asplenium ruta-muraria). The young fronds are easily confused with Cardamine spp., however the mature plant has a delicate beauty all of its own.

Asplenium ruta-muraria

Outside the city centre, all of the above species persist, along with one other. Rustyback (Asplenium ceterach) is so named because of the rust covered scales that cover the underside of the fronds. Its absence from the centre of the city is unclear as it's response to pollution and light intensity (both of which vary detrimentally closer center) is similar to the species mentioned above.

Asplenium ceterach

These fern species lend a beauty and character to the otherwise plain limestone walls on which they grow. Unfortunately many are being removed or treated with herbicide to clean up the walls. Unlike larger, woody plants (such as Buddleja spp.), their rooting and growing does not affect the integrity of the walls and so should be left to decorate the city.

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